Coaxial electrical conductor



Aug. 13, 1940. Y s. RUBEN 2,211,584

COAXIAL ELECTRICAL CONDUCTOR I 7 Filed Oct. 9, 1937 x g I 42 Condunlfwecoazfz g INVENTOR Jamaal Fam' ATTORNEY Patented Aug. 13, 1940 UNITEDSTATES PATENT OFFICE 9 Claims.

This invention relates to coaxial electrical conductors.

An object of the invention is to provide improved flexible coaxialconductors.

A further object is the provision of a unitary structure comprising aninsulated conductor having a flexible conductive sheath over theinsulating surface.

A further object is the provision of a coaxial ,cable having a centralinsulated conductor and 'a second conductor of finely divided metalbonded to the surface of the insulation.

Still another object is the provision of coaxial conductors in which oneof the conductors comprises finely divided metal rendered conductive inplace upon an insulated conductor. 1

Another object is the provision of a method for applying a conductivesurface to an insulated wire.

Other objects will be apparent from the disclosure and from the drawingin which:

Fig. 1 illustrates a method and apparatus for providing a coaxialconductor on an insulated wire, and v Fig. 2 is a view in perspective,partially cut away, of a coaxial conductor showing the central conductorcore, the insulation and a second convaporized metal is sputtered uponthe insulator ductor over the insulation.

The invention comprises the use of a conductive base, such as a copperwire, an insulation on said wire, such as enamel, rubber, etc., and acoating of finely divided metal held on said insulation by a binder, themetal particles being normally non-' conductive, due to inter-particlecontact resistance, and a method for eliminating the interparticleresistance and rendering the finely divided metal conductive over theentire surface. The

,metallic conductive coating produced thereby may be further improved byelectro-plating or electrodepositing of additional metal thereon.

Heretofore several methods have been used to provide electricallyconductive coatings on insulating or high resistance bases. By onemethod atomized metal is sprayed onto the insulator surface by a Schoopgun. By another method surface. Such methods are expensive inasmuch asthey require the use of hydrogen, vacuum or inert atmospheres and thecoatings produced are in a great many cases unsatisfactory due torigidity and lack of uniformity. Another method,

much used, is to graphitize the non-conductive surface and electroplatea metal over the graphite. While for a number of uses this method isffsatisfactory, the specific resistance of thecarbon or graphite is sohigh as to preclude the initial use of high current densities in theelectro-plating process. In addition, as the graphite is not bonded tothe insulator, peeling sometimes results with consequent dislodglng ofthe plated 5 metal film.

The coating produced by my present method results in a low resistancesurface, the conductivity of which closely approximates that of metalfoil.

In an endeavor to produce a coaxial cable utilizing an insulated wireand an auxiliary or second conductor over the insulation which would befree of the limitations heretofore encountered in the art, I have foundthe following:

If the insulated wire is coated with a thin layer of metal powder, suchas copper bronze powder, even though a very small amount of binder isused, insufficient to completely surround the particle surfaces, thecoating is practically non- 20 conductive, the resistance being of anunusably high value. This high resistance I have discovered is due tothe additive effect or sum of the high contact resistance which existsbetween the particles. Further, I have discovered that the 25non-conductivity or high resistance of these coatings of copper orcopper alloys, such as bronze powder, is apparently due to a superficiallayer of cuprous and other oxides on the surfaces of the particles andthat if this oxide layer is re- 30 moved as by reducing and dissolvingthe cuprous oxide, the inter-particle contact resistance is eliminatedand a highly conductive surface is obtained. One method is to coat theinsulated wire with copper bronze powder to a thickness of 3 about mil,using a quick drying binder, such as a solution of 2 ounces of nitrocellulose in one gallon of amylacetate. This solution forms a porousskin between the particles and allows them to be treated. After thecoated film is dried it is exposed to a strong solution of hydrochloricacid, then washed in hot water to remove excess acid or by-products andthereafter dried and heated to eliminate any absorbed moisture or 45methacrylate, etc.

acid, other solutions may be used, for instance, mercurous chloride,mercurous nitrate, hydro- ,fiuoric acid or lactic acid. However, thehydrozchloric acid gives the best results and is preferred.

5 Where mereurous chloride or mercurous nitrate are used, a slightamalgamation takes place between the bronze and the mercury, in additionto the reducing or dissolving action effected.

I have also'found it possible to eliminate the metal powderinterparticle resistance by exposing the coated wire to the vapors givenoff by warm hydrochloric acid. This is readilyaccomplished by passingthe wire through a closed chamber above a vessel containing heatedhydrochloric acid, the reducing or dissolving action being due to thehydrogen chloride gas derived from the acid. After the exposure whichmay be from one to five minutes, depending upon the temperature of theacid, the wire is heated to bring about a further reduction inresistance and drive off any surface absorbed gas or vapors.

The co-axial conductors thus produced are suitable for many uses.However, in order to minimize abrasion effects, facilitate soldering andreduce the possibility of corrosion I prefer to electro-plate a film ofmetal over the coated bronze surface. This may be readily accomplishedat a higher current density than is possible in the electro plating ofmetals upon graphitized or carbonized surfaces, the low resistance ofthe sprayed surface making the immediate use .of high currentspracticable. In part, this superiority to the usual carbon or graphitesurfaces heretofore used is due to the more intimate binding of theconductive particles to the insulation,

the water-proof character of the bronze coating and its lowerresistance. Any of the commonly used plating metals may be employed, forinstance, copper, chromium, tin, cadmium, lead,

49 etc. While I have found a solution of 2 ounces of nitro cellulose toone gallon of amylacetate to be a very satisfactory binder, othermaterials may be used, such as shellac, glyptal, meta The amount ofbinder used should be limited so as not to form a complete physicalinsulating layer around the particles, but sufficient to firmly hold themetal particles upon the insulation. In practice, I have used a 50mixture consistingof 2 ounces of nitro cellulose -In order to morecompletely describe the inv vention, reference is made to the drawing.

In Fig. 1, enamel or rubber covered copper wire is passed into vessel 2,containing finely divided bronze suspended in asolution of nitrocellulose amylacetate 4. The bronze coated wire passes via pulley 3, upthrough heated dryer 5, via pulley 6, into vessel I, containinghydrochloric acid 8, which reduces the inter-particle resistance. Thetreated wire then passes via pulley 9, around pulley III, into vesselcontaining water I2, a fresh supply of water being continuallymaintainedthrough inlet I4, and outlet IS. The washed wire then passes via pulley.l3, up

through heater and dryer l6, which drives out 1' any residual water oracid and thereafter passes via pulley II, into plating tank I8,containing cadmium plating solution I9, and cadmium electrode 20. Theplating potential is supplied by battery 3|. The plated wire then passesvia pulley 2| to metallic roller 22, which is connected to one side ofbattery 3|, and thereafter passes into vessel 23, containing water wash24, a fresh supply of water being maintained through inlet 26 and outlet21. After this final wash the wire \is passed up through heater anddryer 28, and over top roller 29, to be wound ready for use on mandrel30,

In Fig. 2 copper wire I, is shown with enamel varnish insulation II, andtop metal coating 42, corresponding to the condition of the wire as itis rolled on to mandrel 30.

It is apparent that rubber covered wires containing two or moreinsulating layers and two or more conductors in a single casing can alsobe produced by this method using the conductive sheath as a shield,ground or common return conductor for all of the enclosed wires.

The coating produced by the above method is highly conductive, flexible,adherent and well bonded to the insulation.

Having described my invention, what I claim as new and desire to secureby Letters Patent, is:

1. Coaxial electrical conductors comprising a conductor core, aninsulated coating thereover and a conductive coating thereon of copperbronze metal particles bonded to said insulation by an organic binder,said particles having initially high resistance surface layers ofsuperficial thickness so that said coating is substantiallynon-conductive along its length, said high resistance layers beingchemically reduced in situ on said base insulation to a highlyconductive state.

2. Coaxial electrical conductors comprising a conductor core, aninsulating coating thereon and an electrically conductive surface bondedthereto comprising copper base metal particles and a waterproof organicbinder, said surface being initially characterized by highinter-particle contact resistance at the particle interfaces so as tomake said surface non-conductive along its length, said high contactresistance being eliminated by chemical reduction at the particleinterfaces in situ on said base insulating coating, and a metal layerelectroplated upon said particles.

3. The method of adding a conductive surface to an insulated wire whichcomprises coating said insulated wire with copper-bronze particles and acarrier therefor adapted to permanently bond said copper-bronzeparticles to said insulated wire, said copper-bronze particles havingnormally a high resistance layer of superficial thickness at thesurfaces thereof, subjecting said coated insulated wire to the action ofa reducing acid capable of reacting withcopper-bronze particles toconvert said high resistance surfaces to a conductive state.

4. The method of adding a conductivesurface to an insulated wire whichcomprises coating said insulated wire with copper-bronze particlessuspended in a carrier adapted to permanently bond said copper-bronzeparticles to said insulated wire, subjecting said coated insulated wireto the action of an acid in vapor form capable of reacting with saidbronze particles to convert the initial high resistance surfaces of saidbronze particles to a conductive state, heating said treated coatedinsulated wire to drive off acid vapor and moisture and thereafterelectroplating another metal layer upon said coated insulated wire,

5. Themethodof adding a conductive surface to an insulated wire whichcomprises coating said I insulated wire with finely divided copper baseparticles and a permanent organic binder, drying said coating upon saidinsulated wire, said coating being characterized by high contactresistance at the particle interfaces, reacting hydrochlorlc acid withsaid interfaces to bring about high inter-particle conductivity.

6. The method of producing a conductive surface upon an insulated wirewhich comprises applying to an insulated wire a coating of finelydivided copper bronze powder and a permanent organic binder therefor,said coating being substantially non-conductive along its length due tohigh resistance between the bronze particles, subjecting said coating tothe action of hydrochloric acid in vapor form to reduce saidinter-particle resistance and causing said coating to become conductivealong its length.

7. The method of producing a conductive surface upon an insulated wirewhich consists in applying acoating of powdered copper bronze and apermanent organic binder to an insulated wire, said coating'at thisstage being substantially non-conductive along its length, andthereafter subjecting said coating to the action of hydrochloric acid tocause said coating to become conductive along its length.

tion of a reducing acid to convert initial high resistance surfaces ofsaid bronze particles to a conductive state.

9. An electrical conductor comprising a highly conductive metal core, aninsulating coating and an electrically conductive surface bonded theretocomprising copper-base metal particles and a water-proof organic binder,said surface being initially characterized by high inter-particleresistance at the particle interfaces so as to make said surfacenonconductive along its length, said high contact resistance beingeliminated by chemical reduction at the particle interfaces in situ onsaid base insulating coating and a metal I layer electroplated on saidparticles.

SAMUEL RUBEN.

